The Prema Racing driver crossed the line 13 seconds ahead of second-placed title rival Theo Pourchaire in a dominant performance.
The Dane endured an early safety car and was unchallenged thereafter to take his third win in four rounds.
Pourchaire lined up third for ART and though he briefly fell back to fourth, made it back to second to finish ahead of Jack Doohan (Virtuosi).
It was far from easy for the Australian, who endured a fierce battle with Ollie Bearman (Prema) which culminated in the Briton running off at Vale, with Doohan appearing to wave at his rival as he passed.
The race started behind the safety car amid wet conditions, with heavy rainfall having preceded the session.
Though the safety car did pull into the pits, it was deployed again soon after as PHM driver Brad Benavides was nudged into a spin by Clement Novalak, making contact with Ayumu Iwasa who had dropped down the order.
The Trident driver was handed a five-second penalty for the incident, while DAMS driver Iwasa pitted with an issue, though he later rejoined.
Racing resumed on lap four, with Bearman on the charge as he passed Pourchaire for third up the inside of Turn 3 before claiming second from Hadjar at Stowe.
It wasn’t to last though, as Hadjar retook second on lap seven after a spin from Bearman, with Pourchaire then taking back third at the exit of Club the following tour.
Source: Autosport